This invention relates to drive circuits for power MOSFET switching devices and more specifically relates to a novel transformer isolated driver circuit which permits the use of very large duty cycle ratios.
Transformer coupling of low level signals to semiconductor power switching devices offers several advantages such as impedance matching, d-c isolation and step-up or step-down capability. Transformers, however, can deliver only a-c signals since the core flux must be reset each half cycle. A constant number of volt seconds must be reset in both positive and negative half cycles so that very large voltage swings occur in the output winding of the isolation transformer if a large duty cycle is required. That is, if there is a very narrow reset pulse, the voltage induced in the output winding is extremely high in order to produce the necessary volt second area required for resetting the transformer flux. This would exceed the relatively sensitive voltage limitations of a power MOSFET device. Therefore, transformers in semi-conductor drive circuits are usually limited to a 50% duty cycle so that roughly equal pulse widths are used for the positive and negative half cycles.
Thus, when large duty cycle ratios have been needed for a semi-conductor power switch, it is common to employ optical coupling between the input and output signals to provide the necessary drive isolation. Optical couplers, however, have poor noise immunity and a high impedance output. Moreover, they require additional floating power sources which add complexity to the driver circuit. When the power semi-conductor being driven is a power MOSFET device such as the HEXFET power MOSFET device sold by the International Rectifier Corporation of El Segundo, Calif., the high output impedance of the optical coupler is not a serious problem because the HEXFET power MOSFET does not require drive current in the on or off state. However, even in such power MOSFET devices, switching speed will be seriously compromised if a high impedance driver is used.